Power amplifiers are used in applications such as wireless cellular handsets. Wireless cellular devices may implement technologies that typically need particular power requirements. Traditionally, power amplifiers have used GaAs and/or SiGe bipolar transistor technologies which have available high voltage breakdown devices.
It is ideal to integrate power amplifiers with transceiver and baseband circuits as a single chip; however, due to voltage scaling in nanometer scale (nanoscale) technology, the amount of power that a single transistor can deliver typically is quite limited (e.g., 8 dBm).
Since there are limitations in scaling, output resistance, and in order to deliver the same amount of output power that a power amplifier can deliver, multiple transistors or micro power amplifiers can be designed and a power combiner can be used to combine the output power from each micro power amplifier; however, consideration is to be made to assure that the final product has high efficiency. In other words, a power combiner should be power efficient and have low loss. The power combiner would typically be realized as a transformer network, in which a desired individual transformer would require high coupling factor and high inductances on the primary and secondary windings. Manufacturing of such a low-loss, compact and highly efficient transformer network in a large-volume low-cost nanoscale CMOS process and associated packaging technologies would be very difficult to realize.
An additional component that significantly affects the transmitter power efficiency is a frequency band switch coupling the power amplifier(s) to the antenna. To improve power transfer efficiency at RF frequencies, a typical power amplifier must be tuned to a relatively narrow frequency range (e.g., 10-100 MHz centered around 900 MHz or 1800 MHz) with an LC matching circuit. To cover various frequency bands that a handset is specified to cover, multiple power amplifiers or multiple output stages, each terminated with its own matching circuit, are employed. Selecting the desired PA output is a difficult task as it requires expensive high-power-rated switches, typically implemented in pHEMPT technology. The non-zero resistance of the switches contribute 0.5-1 dB of power loss, therefore their elimination would greatly improve the overall TX efficiency. The switches also have finite isolation between ports, therefore their elimination would help to reduce leakage and thus improve dynamic range.
Furthermore, varying antenna loading environment becomes more and more critical in today's environment of ever decreasing form factor of handset cell phones. The antenna load mismatch results in more reflected power and thus less power emitted by the antenna. Therefore, it is desirable to be able to compensate for the antenna mismatch with a controllable matching circuit.